Screen for core legs for transformers, reactors and the like

ABSTRACT

In order to prevent the occurrence of glow discharge at sharp corners and edges of core legs for transformers, reactors and the like which are formed of a plurality of sheets, tape is wound around the bundle of sheets, the tape being formed of a backing of glass fiber or the like having high mechanical strength and impregnated with an electrically semi-conducting and thermosetting material, the successive turns of tape overlapping, the winding being under high tension, the screen after heat treatment and curing being quite stiff and having good mechanical and electrical contact with the core leg along its longitudinal edges.

United States Patent Andersson et a].

SCREEN FOR CORE LEGS FOR TRANSFORMERS, REACTORS AND THE LIKE Inventors: Olle Andersson; Ellerth Ericsson,

both of Ludvika, Sweden Assignee: Allmanna Svenska Elektrika Aktiebolaget, Vasteras, Sweden Filed: Aug. 6, 1974 Appl. No.: 495,1 12

Foreign Application Priority Data Aug. 23, 1973 Sweden 73114639 U.S. Cl. 336/84; 252/511; 310/196 Int. Cl. H01F 15/04 Field of Search 336/84, 210; 310/196;

References Cited UNITED STATES PATENTS 4/1946 Baker et al. 310/196 X 2,705,292 3/1955 Wagenseil 310/196 Primary Examiner-Thomas J. Kozma ABSTRACT In order to prevent the occurrence of glow discharge at sharp corners and edges of core legs for transformers, reactors and the like which are formed of a plurality of sheets, tape is wound around the bundle of sheets, the tape being formed of a backing of glass fiber or the like having high mechanical strength and impregnated with an electrically semi-conducting and thermosetting material, the successive turns of tape overlapping, the winding being under high tension, the screen after heat treatment and curing being quite stiff and having good mechanical and electrical contact with the core leg along its longitudinal edges.

3 Claims, 2 Drawing Figures SCREEN FOR CORE LEGS FOR TRANSFORMERS, REACTORS AND THE LIKE BACKGROUND OF THE INVENTION l. Field of the Invention The invention relates to core legs for transformers, reactors and the like.

2. The Prior Art Close to the electrodes in an electrode gap there are risks of glow discharge occurring when the gap is under tension if the electrode surfaces show irregularities, sharp edges, and the like. One type of electrode where this phenomenon frequently and easily occurs is the transformer-core. One reason for this is that the core legs of a transformer for strictly practical reasons cannot be made with purely circular cross-section, but its circumference is formed from a number of longitudinal surfaces which are perpendicular to each other and with sharp edges at their intersections. Another reason is that the core is manufactured of shorn sheet and that there are always burrs along the shearing edges. Particularly at the edges of the core these burrs will form starting points for glow phenomena when the transformer is put under tension and an electric field arises between the innermost winding and the core leg.

SUMMARY OF THE INVENTION The present invention relates to a screen for core legs BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawing FIG. 1 shows a cross-section through a core leg of a transformer core which is provided with a screen according to theinvention.

FIG. 2 shows how to apply the screen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the invention, the core leg of a transformer or a reactor is providedwith a glow preventing screen of a material which, to a limited degree, is conductive, that is, is semiconducting. When using the material in oil-filled apparatus it .must be able to endure the high temperatures occurring both during operation and during the drying process. The screen is manufactured of a tape 1 or the like with a structure or backing having great mechanical strength and is impregnated with an electrically semiconducting and thermosetting material. The tape is wound with a good mechanical tension and with overlapping to make the screen complete. The degree of overlapping is determined by the thickness of the tape and the desired thickness of the finished screen. As is clear from FIG. 1, the screen will make contact with every protruding edge 3 of the core leg 2, and, since the tape is wound on so as to be well stretched, the tape as well as the finished screen will be in perfect mechanical and galvanic contact with the .core leg along all protruding edges of the leg.

The structure or backing of the tape may, for example, consist of glass fiber fabric giving the tape a very high mechanical strength. The impregnating agent may contain resins of miscellaneous types, such as unsaturated polyesters and epoxy resins.

A few examples of the composition of suitable impregnating agents are given in the following:

EXAMPLE 1 33 parts by weight of an unsaturated polyester manufactured in a conventional manner of 1 mole maleic acid, 2 moles isophthalic acid and 3 moles 1,2-propane diol (acid value 30).

27 parts by weight of diallyl phthalate monomer,

40 parts by weight of graphite powder 250 mesh,

0.8 parts by weight of tertiary butyl perbenzoate.

The components are mixed together to form a homogeneous compound. This compound is applied to the glass tape on one or both sides. After curing the compound is electrically semiconducting.

EXAMPLE 2 EXAMPLE 3 60 parts by weight of an epoxy novolac with an epoxy chemical equivalent of l76l8l and a viscosity of 20-50 l0 cps at 52C,

1.5 parts by weight of amin-BF -complex compound (for example BF-MEA from Allied Chemical Corp.),

18 parts by weight of soapstone powder,

20 parts by weight of graphite flakes.

The components are mixed together to form a homo- I geneous compound. This compound is applied to the tape on one or both sides. After curing the compound is electrically semiconducting.

EXAMPLE 4 30 parts by weight of an epoxy novolac with an epoxy chemical equivalent of 176-181 and a viscosity of 20-50 1O cps at 52C,

1 part by weight of amin- BF -complex (for example BF-MEA from Allied Chemical Corp),

30 parts by weight of graphite powder 3-4 microns, 88-92% C.

The components are mixed together into a homogeneous compound. The compound is applied to the glass tape on one or both sides. After curing the compound is electrically semiconducting.

' EXAMPLE 5 50 parts by weight of a liquid epoxy resin, of bisphenol A type, with chemical equivalent i 3 and viscosity about 12,000 cps at 25C,

50 parts by weight of hexahydrophthalic acid anhydride,

0.5 parts by weight of benzyl dimethylamine,

25 parts by weight of conductive carbon black, 99%

The components are mixed together into a homogeneous compound. The compound is applied to the tape on one or both sides. After curing the compound is electrically semiconducting.

EXAMPLE 6 30 parts by weight of a polybutadiene resin with a vinyl content of at least 90 percent and a mole weight of at least 20,000,

3 parts by weight of ditertiary butyl peroxide,

30 parts by weight of graphite powder 3-4 microns, 83-85% C,

40 parts by weight of wollastonite.

The components are mixed together into a homogeneous compound. The compound is applied to the tape on one or both sides. After curing the compound is electrically semiconducting.

The problem in the present case is to choose the maf terial components in the semiconducting impregnating agent with regard to the resistivity and the dielectric constant so that, on the one hand, the losses in the screen, caused by the currents having the operating frequency, are negligible in comparison with the iron losses in the leg and, on the other hand, a sufficient screening effect is obtained for the steepest voltage wave.

The upper permissible resistivity limit is determined by the capacity of the material to screen during transient phenomena, whereas its lower permissible limit is determined by the losses in the screen which can be accepted.

Because of the fact that the resistivity of the tape and thus of the finished screen is changed with the properties, grain size, and so on, of the semiconducting material, exact information asthe amount of semiconducting materials cannot be given. As a general rule for the choice of the electric properties of the tape the following formula can apply:

ID g the lowest permissible surface resistivity of the screen considering the losses therein (Q square) 8 thickness of screen p, resistivity of screen material a relative dielectric constant of screen material 6 dielectric constant for vacuum T the highest permissible electric time constant of screen material. This is determined from a comparison with the rise time T for the steepest surge volt age for which the screen is to have an acceptable screening effect.

Because of the fact that the transformer core legs are manufactured by applying a large number of thin sheetmetal strips on top of each other (interleaving) the leg can never be quite straight in practice, but local irregularities will arise. It is therefore impossible to place a prefabricated circular screen around the core leg in such a way that the screen will be in galvanic contact with the edges of the core leg all around. In relatively large and randomly scattered areas there will be no contact between the screen and the leg, and the security against glow discharge can never be assured. The same phenomena also make it impossible to apply a prefabricated circular cylindrical screen around the leg to hold the sheets of the core leg together mechanically, since such a screen would never be in adequate mechanical contact with the core leg.

Because the screen is manufactured as described above, it will make an absolutely safe mechanical and electrical contact with all the protruding edges of the leg over its entire length. Since the tape is wound with a certain mechanical tension, which is of course kept so high that the desired mechanical contact pressure against the leg is obtained, it is assured that this contact will be permanent.

The previously mentioned mixtures, used' as impregnating agents, are strongly sticky in unhardened condition. This gives the tape the favorable property that, if it is wound with overlapping and with tensile stress, the different turns will stick to each other very strongly. As a matter of fact the stickiness is so pronounced that, if the winding is interrupted and the tension ceases, the part of the tape which has made contact with an underlying layer of the tape or the iron core will remain. If shrinkage occurs it will be quite negligible. This is valuable both in large iron cores, where a great length of tape is required and where whole tapes in one piece are not available, and in smaller cores where the smaller distance between two core legs may involve certain difficulties of threading the tape through the opening between two core legs during winding.

To avoid unnecessary heating of the iron core, the hardening of the resin in the wound screen is suitably performed by means of infra-red radiation. For this a tubular furnace can be used, surrounding the leg and the screen. The furnace consists of a circular casing carrying on the inside a number of electrically driven infrared radiators which are directed towards the center of the furnace. The infrared rays are radially displaceable so that the furnace may be used for different screen diameters. The whole furnace is axially displaceable and its axial movement can be controlled with timing devices to ensure that the curing time is the same all over. The whole curing process can therefore be carried out automatically after starting and adjusting of the axial movement of the furnace. Since it is normal to use graphite as semiconducting ingredient in the impregnating agent, the tape is black in color. This causes the tape to absorb the infrared rays and the heating of the iron core is slight as there is an air gap between the screen and the core behind it (FIG. 1 Curing of resins in general by means of infrared radiation is well known. In this connection, however, said heating process is necessary because of the great thermal capacity of a core leg.

Using suitable equipment, the tape can be wound on the core leg by hand or in a machine for compression of the sheet-metal packets. The work can be performed with the core leg in vertical or horizontal position.

The uniting properties of the screen can be further strengthened by usinga tape which shrinks during the curing. A pressure on the screen which increases during the curing can also be obtained by winding a shrinkable foil outside the screen prior to curing. When the foil shrinks in response to heating, an increased pressure on the core leg is obtained and, in addition, the screen acquires a smooth and even outer surface.-

We claim:

1. A core leg for transformers, reactors and the like formed of a plurality of magnetic sheets with a surrounding tape to prevent the occurrence of glow disnated with an impregnant consisting essentially of an electrically semiconducting and thermosetting material, wound around the sheets with high mechanical tension and with overlapping so that a screen surrounding the core leg (2) is formed, said screen being cured by heat treatment and being quite stiff and having good mechanical and electrical contact with the core leg at least along the longitudinal edges (3) of the core leg.

2. A core leg according to claim 1, in which the backing of the tape (1) consists essentially of a fabric of glass fiber.

3. A core leg according to claim 1, in which the material properties of the tape can be expressed in the formula:

R the lowest permissible surface resistivity of the screen considering the losses therein (Q/square) 8 thickness of screen I p resistivity of screen material 6 relative dielectric constant of screen material t dielectric constant for vacuum T the highest permissible time constant of screen material, which is determined out of a comparison with the rise time T for the steepest surge voltage for which the screen is to have an acceptable screening effect. 

1. A core leg for transformers, reactors and the like formed of a plurality of magnetic sheets with a surrounding tape to prevent the occurrence of glow discharge at sharp corners and edges where there prevail inhomogeneous electric fields and to hold the core sheets together mechanically, said tape (1) comprising a backing having high mechanical strength and impregnated with an impregnant consisting essentially of an electrically semiconducting and thermosetting material, wound around the sheets with high mechanical tension and with overlapping so that a screen surrounding the core leg (2) is formed, said screen being cured by heat treatment and being quite stiff and having good mechanical and electrical contact with the core leg at least along the longitudinal edges (3) of the core leg.
 2. A core leg according to claim 1, in which the backing of the tape (1) consists essentially of a fabric of glass fiber.
 3. A core leg according to claim 1, in which the material properties of the tape can be expressed in the formula: 